Domain-Driven Design SWEN-261 Introduction to Software Engineering - - PowerPoint PPT Presentation

SWEN-261 Introduction to Software Engineering

Department of Software Engineering Rochester Institute of Technology

Domain-Driven Design

Domain driven design centers the architecture on the problem domain.

• Quote from the DDD Community:

Domain-driven design (DDD) is an approach to developing software for complex needs by deeply connecting the implementation to an evolving model of the core business concepts

• The premise:
• Place the project’s primary focus on the core domain

and domain logic

• Base complex designs on a model
• Initiate a creative collaboration between technical

and domain experts to iteratively cut ever closer to the conceptual heart of the problem

2

Let's review our project architecture.

3

Client UI Application Model

HTML, CSS & JavaScript Java Web server (Jetty) Any OS and HW Any Browser Any OS/HW

Server UI

Spark & FreeMarker

Frameworks Platform OS/Hardware Network Connection

What goes in these two tiers?

DDD provides guidance for the remaining tiers.

4

Client UI Application Model

HTML, CSS & JavaScript Java Web server (Jetty) Any OS and HW Any Browser Any OS/HW

Server UI

Spark & FreeMarker

Frameworks Platform OS/Hardware Network Connection

Services provide application logic.

• The Application tier is responsible for managing

the user's interaction with the application.

• It is not responsible for domain logic which is in

the Model layer.

• Application tier elements provide services to each

client connection.

• Manage application-wide logic and information
• Provide client-specific services for the UI tier

5

Entities provide domain logic.

• The Model tier is responsible for managing

domain entities and domain logic.

• Entity responsibilities are:
• Process user requests/commands
• Effect changes based on user requests/commands
• Validate Model-tier rules
• Maintain the state of the Model
• Entities often represent information about the

world, and are inspired by domain model entities

• Customers, products and orders in e-commerce
• Shapes in a drawing app

6

Value objects provide values for an entity's "complex" attributes.

• A value object class encapsulates the data that

represents an entity's attribute.

• Measurements, dates, credit card numbers, money,

colors, (x,y) coordinates are some examples.

• Two value objects are equal based on equality of the

data in the object not object identity.

• Value objects must be immutable.
• An address of 15 N. Main St cannot be changed into

352 2nd Ave.

• You create a new address object of 352 2nd Ave.
• A value object class is not just a data holder class.
• Value Object = "value semantics" + immutability +

GRASP Information Expert, ref. Flight class in OOD I

7

Model objects are frequently used in collections.

• Many of the algorithms used in Model and

Application components require using Entities and Value Objects in hash-based collections.

• Normal Java equality semantics are not adequate

when dealing with Entities and VOs

• An Entity must have a distinct id such that two
• bjects with the same id must be considered equal.
• Two Value Objects with the same data must be equal.
• These semantic requirements imply specialized

equals and hashCode methods.

• The after-class exercise provides instructions on

how to create these methods.

8

A semantically correct value object can be used as a key in a map collection.

• Rather than extracting attributes from the value
• bject to create a key, use the value object directly.
• This will work correctly because
• The value object is immutable  other code with a

reference to the object can not change the object's value while it is in the map as a key

• The equals and hashCode methods ensure that two
• bjects with the same value will be considered equal

and generate the same hash code.

9

Let's review the architecture again.

10

This is the list of component responsibilities.

11

UI Tier Application Tier Model Tier UI Controller:

• Control the views based on the state of the

application

• Query the Model and Application tier as

necessary to get information to present to the user

• Perform simple input validation and data

conversion based on input modality, e.g. String to integer

• Initiate processing of user

requests/commands possibly providing data the user input

• Perform data conversion for display by views

UI View:

• Provide an interface to the user
• Present information to the user in a variety
• f ways
• Provide a mechanism for user to input data

and requests

Service:

• Manage application-wide

logic and information

• Provide client-specific

services to the UI tier

Entity:

• Process user requests/commands
• Effect changes to the Model based
• n user requests/commands
• Validate model rules
• Maintain the state of the model

Value Object:

• Provide immutable value

semantics

• Provide value-based logic